Light emitting semiconductor element with ZN doping

ABSTRACT

An LED is disclosed which has a laminated semiconductor body with an anode and a cathode formed on a pair of opposite faces thereof. Among the layers of the semiconductor body are an active layer of AlGaInP semiconductor material, an n type cladding layer of either n type AlGaInP or n type AlInP semiconductor material on one side of the active layer, and a p type cladding layer of p type AlGaInP or p type AlInP semiconductor material on another side of the active layer. Unlike the conventional belief that the active layer should be as free as possible from the infiltration of Zn used in the p type cladding layer as an impurity to determine its p conductivity type, and hence as high in crystallinity as possible, Zn is positively doped into the active layer with a concentration of 1×10 16  -5×10 17  cm -3 .

BACKGROUND OF THE INVENTION

This invention relates to light emitting semiconductor elements, orlight emitting diodes (LEDs) according to more common parlance, whichserve as light sources when voltage is applied and which thus lendthemselves to use for a variety of display or illumination purposes.

LEDs have been known and used extensively which have a multi-layersemiconductor body sandwiched between an anode and a cathode. Thepresent invention has particular pertinence to those LEDs whosesemiconductor body is a lamination of an n type gallium arsenide (GaAs)semiconductor substrate, an n type GaAs semiconductor buffer layer, an ntype aluminum indium phosphide (AlInP) cladding layer, an aluminumgallium indium phosphide (AlGaInP) semiconductor active layer, a p typeAlInP semiconductor cladding layer, a p type aluminum gallium arsenide(AlGaAs) semiconductor current spreading layer, a p⁺ AlGaInPsemiconductor protective layer for prevention of oxidation, a p typeGaAs semiconductor contacting layer, and an n type AlGaInP semiconductorcurrent blocking layer. An anode has been formed next to the contactinglayer, and a cathode next to the substrate. Light is emitted from thatpart of the p⁺ type AlInP semiconductor cladding layer which is leftuncovered by the anode.

Typically, the n type AlInP semiconductor cladding layer has been dopedwith silicon as an impurity that determines the n conductivity type. Thep type AlInP semiconductor cladding layer has been doped with zinc (Zn)as an impurity that determines the p conductivity type. Although itselfnot doped with Zn, the AlGaInP semiconductor active layer was partlyinfiltrated with the Zn that has been doped into the neighboring p typecladding layer. Usually, the Zn infiltration into the active layer hasbeen to a depth of 0.1 micrometer or so, or up to one fifth of thethickness of the active layer.

The conventional belief was that the Zn presence in the active layer didharm to its crystallinity, increased recombinations that did not resultin light emission, and thus lowered the efficiency of the complete LED.Consequently, the active layer was of course not positively doped withZn, and attempts were made to reduce the Zn infiltration from the p typecladding layer to a minimum. Despite such conventional efforts tomaintain the crystallinity of the active layer, the LEDs of this kindwere not as high in efficiency as could be desired.

SUMMARY OF THE INVENTION

The present invention seeks to improve the efficiency of the lightemitting semiconductor elements or LEDs of the kind defined.

Briefly, the invention may be summarized as a light emittingsemiconductor element having a semiconductor body having a plurality oflaminated semiconductor layers for emitting light, a cathode on one faceof the semiconductor body, and an anode on another face of thesemiconductor body. More specifically, the invention resides in thesemiconductor body consisting essentially of an active layer of AlGaInPsemiconductor material additionally having Zn approximately uniformlydispersed therein with a concentration of 1×10¹⁶ -5×10¹⁷ cm⁻³, theactive layer being 0.5-2.0 micrometers thick, an n type cladding layerof either n type AlGaInP or n type AlInP semiconductor material on oneside of the active layer, and a p type cladding layer of p type AlGaInPor p type AlInP semiconductor material on another side of the activelayer.

The noted three semiconductor layers are given as essential parts of thesemiconductor body in the fabrication of light emitting semiconductorelements according to the invention. Various other semiconductor layersmay be added as required.

Experiment has proved that the positive doping of Zn into the activelayer within the specified range of concentrations according to thepresent invention, as distinguished from accidental Zn infiltration intolimited part of the active layer according to the prior art, materiallyreduces the amount of light emitted at wavelengths longer than thedesired ones and, as a result, correspondingly increases the amount oflight at the desired wavelengths.

Admittedly, the Zn doping adversely affects the crystallinity of theactive layer and thus lessens the total amount of light produced.However, as far as the desired wavelength light is concerned, its amountis increased through the suppression of light emission at longerwavelengths, instead of being decreased through the drop incrystallinity.

The above and other features and advantages of this invention and themanner of realizing them will become more apparent, and the inventionitself will best be understood, from a study of the followingdescription and appended claims, with reference had to the attacheddrawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of an LED constructed inaccordance with the present invention;

FIG. 2 is a graph plotting the curve of the relative intensity of thelight produced by the noted prior art LED against the wavelength; and

FIG. 3 is a graph plotting the curve of the relative intensity of thelight produced by the FIG. 1 LED according to the invention against thewavelength.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described more specifically in termsof the LED illustrated in FIG. 1 by way of a representative embodimentof the invention. Broadly, the representative LED is a combination of asemiconductor body 10 having a pair of opposite faces, an anode 11 onone face of the semiconductor body, and a cathode 12 on the other faceof the semiconductor body.

The semiconductor body 10 is a lamination of an n type GaAssemiconductor substrate 1, an n type GaAs semiconductor buffer layer 2,an n type AlInP semiconductor cladding layer 3, an AlGaInP semiconductoractive layer 14, which additionally has Zn uniformly dispersed thereinaccording to the novel concepts of the present invention, a p type AlInPsemiconductor cladding layer 5, a p type AlGaAs semiconductor currentspreading layer 6, a p⁺ type AlGaInP semiconductor protective layer 7for prevention of oxidation, and a p type GaAs semiconductor contactinglayer 8 of smaller size, in that order from cathode 12 toward anode 11.

Additionally, an n type AlGaInP semiconductor current blocking layer 9,which is also less in size than all but the contacting layer 8 of theconstituent layers of the semiconductor body 10, is interposed betweencladding layer 5 and current spreading layer 6. Both contacting layer 8and current blocking layer 9 are disposed centrally of the semiconductorbody 10 as seen in a plan view. Thus the current blocking layer 9contacts with only part of one side surface of the p type cladding layer5, and the current spreading layer 6 contacts with both cladding layer 5and current blocking layer 9.

The anode 11, which is made from gold (Au), is of approximately the samesize as contacting layer 8 and overlies the same. Light is thereforeemitted from that part of the protective layer 7 which is left uncoveredby the anode 11.

Given hereafter is a more detailed description of the constituent layersof the semiconductor body 10. The substrate 1 is 330-370 micrometersthick and contains an impurity that determines its conductivity type, ata concentration of 1×10¹⁸ -4×10¹⁸ cm⁻³.

The buffer layer 2 is 0.1-0.3 micrometer thick and contains an impuritythat determines its conductivity type, at a concentration of 1×10¹⁸-5×10¹⁸ cm⁻³.

The n type cladding layer 3 is 0.5-1.5 micrometers thick and contains animpurity that determines its conductivity type, at a concentration of8×10¹⁶ -1×10¹⁸ cm⁻³.

The active layer 14 is 0.5-2.0 micrometers thick and contains Zn at aconcentration of 1×10¹⁶ -5×10¹⁷ cm⁻³.

The p type cladding layer 5 is 0.5-1.5 micrometers thick and contains animpurity that determines its conductivity type, at a concentration of5×10¹⁶ -1×10¹⁸ cm⁻³.

The current blocking layer 9 is 0.05-0.25 micrometer thick and containsan impurity that determines its conductivity type, at a concentration of1×10¹⁸ -1×10¹⁹ cm⁻³.

The current spreading layer 6, known also as the front window layer, is5-15 micrometers thick and contains an impurity that determines itsconductivity type, at a concentration of 1×10¹⁸ -5×10¹⁹ cm⁻³.

The protective layer 7 is 0.2-0.4 micrometer thick and contains animpurity that determines its conductivity type, at a concentration of1×10¹⁷ -1×10¹⁸ cm⁻³.

The contacting layer 8 is 0.05-0.15 micrometer thick and contains animpurity that determines its conductivity type, at a concentration of5×10¹⁸ -7×10¹⁹ cm⁻³.

For the fabrication of the semiconductor body 10 of the foregoingconfiguration, there may first be conventionally prepared a combinationof the substrate 1 and the buffer layer 2, the latter being formed byepitaxial growth of GaAs upon the former. Then, on this buffer layer 2,there may be epitaxially grown the n type AlInP cladding layer 3containing silicon (Si) as an impurity that determines the nconductivity type. Immediately after the formation of the cladding layer3, the impurity may be changed from Si to Zn, and the semiconductorsubstance may be changed from AlInP to AlGaInP for epitaxial growth onthe cladding layer. It is possible in this manner to form continuouslythe cladding layer 3 and the AlGaInP semiconductor active layer 14, thelatter being permeated with Zn throughout its body.

Then the cladding layer 5 may be formed on the active layer 14 byepitaxial growth of p type AlInP. As has been stated, the active layer14 is 0.5-2.0 micrometers thick, which is sufficiently more than thedepth (approximately 0.1 micrometer) of infiltration of Zn from thecladding layer 5. Then n type AlGaInP may be grown epitaxially on thecladding layer 5, and unnecessary part of the resulting product may beremoved to provide the current blocking layer 9. Then the currentspreading layer 6, the protective layer 7 and the contacting layer 8 maybe formed one after another by epitaxial growth of p type AlGaAssemiconductor, AlGaInP semiconductor, and p type GaAs semiconductor.

FIG. 2 graphically represents the intensity of light emitted by theprior art LED, having the active layer 4 not doped with Zn, at variouswavelengths. In this graph, as well as in that of FIG. 3 which similarlydemonstrates the performance of the LED having the active layer 14 dopedwith Zn according to the present invention, the light intensity is givenin percentage with respect to the maximum intensity indicated, which is100 percent. Both prior art and inventive LEDs were of course drivenwith current of the same magnitude in conducting the tests of FIGS. 2and 3. The high intensity light at 570 nanometers and thereabouts inboth FIGS. 2 and 3 is the desired green light.

The irregular intensity rises at 730 nanometers and thereabouts in FIG.2 indicate dark red light due to deep emission according to the priorart. Such undesired emission lowers the efficiency of the LED at thedesired wavelength. It has been confirmed that the intensity of the deepemission increases with an increase in x in the (Al_(x) Ga_(1-x))InPwhich constitutes the active layer, that is, with an increase in theenergy gap of the active layer.

FIG. 3 demonstrates that such deep emission can be virtually eliminatedby uniformly doping Zn Into the active layer 14, resulting in the higherintensity of the light at the desired wavelength. Actually, the lightintensity at the desired wavelength of the LED according to the presentinvention was more than twice that of the prior art device of likeconstruction except that Zn was not uniformly doped Into the activelayer.

Despite the foregoing detailed disclosure it is not desired that theinvention be limited by the exact showing of the drawings or thedescription thereof. The following is a brief list of the possiblemodifications or alterations of the illustrated embodiment:

1. The n type cladding layer 3 and the p type cladding layer 5 couldeach be divided into two sublayers, and the potential barrier betweenthe active layer 14 and that sublayer of each cladding layer which liesnext to the active layer could be made higher than that between theactive layer and the other sublayer of each cladding layer.

2. Both n and p type cladding layers 3 and 5 could be of AlGaInP insteadof AlInP.

3. The Zn concentration of the active layer 14 could be anywhere between1×10¹⁶ cm⁻³ and 5×10¹⁷ cm⁻³, no substantial improvement in efficiencybeing obtainable outside this range.

4. The active layer 14 could be anywhere between 0.5 and 2.0 micrometersin thickness. A drop in efficiency would result at less than 0.5micrometer under the influence of the interfaces with the claddinglayers, and a drop in efficiency would also result at more than 2.0micrometers due to a decrease in carrier concentration of the activelayer 14.

5. It is only the n type cladding layer 3, the active layer 14, and thep type cladding layer 5 that are essential in the illustrated LED, allthe other semiconductor layers being optional.

6. The concepts of the present invention are applicable to theelimination of deep emission that conventionally occurred at other than730 nanometers because of different proportions of aluminum in theactive layer.

All these and other modifications and alterations are intended in theforegoing disclosure. It is therefore appropriate that the invention beconstrued broadly and in a manner consistent with the fair meaning orproper scope of the attached claims.

WHAT IS CLAIMED IS:
 1. A light emitting semiconductor element having asemiconductor body having a plurality of laminated semiconductor layersfor emitting light, a cathode on one face of the semiconductor body, andan anode on another face of the semiconductor body, wherein theimprovement resides in the semiconductor body comprising:(a) an activelayer of AlGaInP semiconductor material additionally containing Zn at aconcentration of 1×10¹⁶ -5×10¹⁷ cm⁻³, the active layer being 0.5-2.0micrometers thick; (b) an n type cladding layer of either n type AlGaInPor n type AlInP semiconductor material on one side of the active layer;and (c) a p type cladding layer of p type AlGaInP or p type AlInPsemiconductor material on another side of the active layer.
 2. The lightemitting semiconductor element of claim 1 wherein the n type claddinglayer is 0.5-1.5 micrometers thick and contains an impurity thatdetermines the conductivity type thereof, at a concentration of 8×10¹⁶-1×10¹⁸ cm⁻³, and wherein the p type cladding layer is 0.5-1.5micrometers thick and contains an impurity that determines theconductivity type thereof, at a concentration of 5×10¹⁶ -1×10¹⁶ cm⁻³. 3.A light emitting semiconductor element comprising:(A) a semiconductorbody of laminar construction comprising:(a) a substrate of n type GaAssemiconductor material; (b) a buffer layer of n type GaAs semiconductormaterial; (c) an n type cladding layer of AlInP semiconductor material;(d) an active layer of AlGaInP semiconductor material additionallycontaining Zn at a concentration of 1×10¹⁶ -5×10¹⁷ cm⁻³ ; (e) a p typecladding layer of p type AlInP semiconductor material; (f) a currentblocking layer of AlGaInP semiconductor material; (g) a currentspreading layer of p type AlGaAs semiconductor material; (h) aprotective layer of p type AlGaInP semiconductor material; and (i) acontacting layer of p type GaAs semiconductor material; (B) a cathodeformed contiguous to the substrate of the semiconductor body; and (C) ananode formed contiguous to the contacting layer of the semiconductorbody.
 4. The light emitting semiconductor element of claim 3 wherein thesubstrate of the semiconductor body contains an impurity that determinesthe conductivity type thereof, at a concentration of 1×10¹⁸ -4×10¹⁸cm⁻³, wherein the buffer layer contains an impurity that determines theconductivity type thereof, at a concentration of 1×10¹⁸ ×5×10¹⁸ cm⁻³,wherein the n type cladding layer contains an impurity that determinesthe conductivity type thereof, at a concentration of 8×10¹⁶ -1×10¹⁸cm⁻³, wherein the p type cladding layer contains an impurity thatdetermines the conductivity type thereof, at a concentration of 5×10¹⁶-1×10¹⁸ cm⁻³, wherein the current blocking layer contains an impuritythat determines the conductivity type thereof, at a concentration of1×10¹⁸ -1×10¹⁹ cm⁻³, wherein the current spreading layer contains animpurity that determines the conductivity type thereof, at aconcentration of 1×10¹⁸ -5×10¹⁹ cm⁻³, wherein the protective layercontains an impurity that determines the conductivity type thereof, at aconcentration of 1×10¹⁷ -1×10¹⁸ cm⁻³, and wherein the contacting layercontains an impurity that determines the conductivity type thereof, at aconcentration of 5×10¹⁸ -7×10¹⁹ cm⁻³.
 5. The light emittingsemiconductor element of claim 3 wherein the substrate of thesemiconductor body is 330-370 micrometers thick, wherein the bufferlayer is 0.1-0.3 micrometer thick, wherein the n type cladding layer is0.5-1.5 micrometers thick, wherein the active layer is 0.5-2.0micrometers thick, wherein the p type cladding layer is 0.5-1.5micrometers thick, wherein the current blocking layer is 0.05-0.25micrometer thick, wherein the current spreading layer is 5-15micrometers thick, wherein the protective layer is 0.2-0.4 micrometerthick, and wherein the contacting layer is 0.05-0.15 micrometer thick.